Networks of store and forward messaging systems such as, for example, networks of store and forward voice and/or data messaging systems are well known in the art. As is well known, such networks are typically comprised of a group of individual store and forward systems which are interconnected by communications links into a network. A user of such a network of store and forward messaging systems is typically, but not necessarily, a human (sometimes referred to as a user) who "owns" a mailbox at the user's local system (sometimes referred to as a local site or a home site). As is well known, a mailbox is a logical abstraction in a store and forward messaging system at which a user's messages are stored and which is seen by the user as a single logical storage area. Users may create messages (a message is a group of information and can be a voice message, a data message, a facsimile message, a combination of voice, data, and facsimile information, and so forth) which are sent from their mailboxes to other mailboxes. Mailboxes on the user's local site are called local mailboxes and mailboxes on other sites are called network mailboxes. Users may receive messages which are transmitted to their mailboxes.
As is well known, a distribution list is a list of members (a member of the distribution list is sometimes referred to as a destination), which members may either be users or other distribution lists. In prior art systems, whenever a message is addressed to a distribution list, the message is redistributed, or not, as the site permits, separately to each destination in the distribution list (a redistribution of messages in a distribution list is sometimes referred to as an expansion of the distribution list). For the purpose of describing the present invention, a message received at one site in the network from another site in the network is referred to as a "network message" and a message which has never left a local site is called a "local message."
In a typical prior art network of store and forward messaging system sites, a user provides the address of a mailbox for message delivery and each site is able to determine whether the mailbox is local or not. Further, each site is able to determine the site in the network to which a message is to be sent if the mailbox is not local. In general, however, a site cannot determine whether a network mailbox refers to a user or to a distribution list. Further, in the case of a distribution list, a site cannot determine whether a member which is a network mailbox refers to a user or to a distribution list.
In a typical prior art network of store and forward messaging system sites, in order to prevent infinite loops of message delivery from occurring, network messages, i.e., messages received at a site from another site in the network, are restricted in that they may only be automatically distributed to users at the reception site. A user who receives a network message can, if the site permits, forward that message to any site in the network, and that message, directed by the user, is treated as a new message. In contrast, a network message addressed to a distribution list can only be automatically expanded to addresses or destinations, i.e., mailboxes, at the local site where the distribution list resides. Further, in such a typical prior art network, a maximum depth of nesting of distribution lists within a site is imposed. Nesting means that a message is sent to a distribution list at a site which redistributes it, in turn, to another distribution list at the same site. However, in accordance with a nesting restriction, if the message reaches the maximum depth of nesting of distribution lists for a site, the message is only further expanded or distributed to users, i.e., sent to users' mailboxes, and not to further distribution lists. The above-described restrictions are used in the prior art to prevent infinite loops which would be caused, for example, by two distribution lists, each containing the other (among, possibly, other destinations) as a destination.
The above-described prior art systems have several disadvantages. In particular, a user who sends messages to recipients who are scattered across such a prior art network can only use a distribution list defined at the user's home site or the destinations local to the site of a distribution list at another site. This is disadvantageous in that it requires duplication of commonly used distribution lists which contain destinations at more than one site. This is further disadvantageous in that administration of such distribution lists, for example, to keep track of changes, grows rapidly with the number of sites in the network. As an example, consider a company which has many sites -with a vice president at each site. In using one of the above-described prior art systems, one would have to set up a separate distribution list for each site to distribute a message to all vice presidents of the company. Maintaining such distribution lists, for example, if one vice president temporarily relocates to a different site or changes his/her address (mailbox number), requires much administrative effort.
A still further disadvantage of the above-described prior art systems is that a user cannot design distribution lists which minimize message transmission costs. To understand this, assume that a user, for example, an engineer, working for a multinational company in the United States of America wishes to send a message to his/her colleagues, for example, an engineering group, at different sites in Europe. The least expensive way to do this is to send a single copy of the message to a site in Europe, where a distribution list would then distribute the message to all the European destinations. This is not allowed because the distribution list would need to expand to destinations at other sites. Instead, the user would use a locally is defined distribution list which would be expanded at his/her local site. As a result, a separate copy of the message would be sent to each of the different sites, thereby incurring a high transmission cost.
In light of the above, there is a need in the art for method and apparatus for use in a network of store and forward messaging system sites which avoid infinite loops while providing user access to distribution lists in the network, the members of which distribution lists are users and distribution lists which may be located at other network sites.